<!-- <!DOCTYPE html>
<html lang="en">

    <head>
        <meta charset="UTF-8"/>
        <meta content="width=device-width, initial-scale=1.0" name="viewport"/>
        <meta content="ie=edge" http-equiv="X-UA-Compatible"/>
        <script src="https://cdn.bootcss.com/stylus/0.32.1/stylus.js"></script>
        <title>Document</title>
        <style>
            .do {
                position: relative;

                &:after {
                    position: absolute;
                    width: 100%;
                    border: 1px solid black;
                    left: 0;
                    bottom: 0;
                }
            }
        </style>
    </head>

    <body>
        <div class="do">你好</div>
    </body>

</html> -->
